1. Field of the Invention
The invention relates to non-woven felt-like products comprised of filamentary polytetrafluoroethylene (PTFE). More particularly, it relates to an improvement in said felt-like products. The improvement is the presence of fibrils of PTFE which criss-cross interstices of the felt-like products.
2. Prior Art
Felts (i.e., non-woven unbonded fibrous structures deriving coherence and strength from interfiber entanglement and accompanying frictional forces) represent the oldest form of textile fabric. Animal fibers, such as wool and, to a degree, fur, are accepted as the only true feltable fibers. Forming them into felts requires preliminary compaction or "hardening" followed by additional working with addition of heat and usually moisture.
Felting of other filamentary materials has only been possible for a relatively short time, and felt-like products composed of them have only been available for a short time. U.S. Pat. No. 2,910,763, granted Nov. 3, 1959, to Herbert G. Lauterbach, discloses these felt-like products and processes for their preparation.
Since Lauterbach's discovery, felt-like products of PTFE have become a common commercial product for a variety of uses, for example, filtration and padding.
When the currently-used felt-like products are used in filtration, a balance between porosity and efficiency must be struck. High porosity of a felt-like product usually indicates that interstices are large. The higher the porosity of currently-used felt-like product, the less efficient it is as a filter because particles will be able to pass through the interstices of the felt.
However, high porosity is desirable because it produces a higher air/cloth ratio capability and causes a low pressure drop across the felt-like product.
These properties will result in longer filtration cycles between cleaning, less energy required for filtration, and longer life of the filter.
However, the high efficiencies required for filtration (above 99% in order to meet standards promulgated by governmental agencies) severely limit the porosity levels useful.
The product of this invention is a felt-like product of PTFE having significantly increased porosity that, when used as a filter, maintains an efficiency of above 95%, preferably above 99%.